The rebuttal:To begin with, it is important to note that the assertion
that the criterion of shishim ribo is conditional of a city is a
relatively new one. Let us explore the core
arguments regarding how to apply the criterion of shishim ribo, to a
city or to a street. The main evidence cited by those who claim that shishim
ribo is conditional on a city is that Rashi, the central supporter
of this fundament, employs the word ir [city] when mentioning shishim
ribo (Eruvin 6a):

ר"ה: משמע רחב שש עשרה אמה
ועיר שמצויין בה ששים רבוא

However, the source for Rashi,
the Behag (Berlin edition, p. 131), refers to a place/road:

רשות הרבים דוכתא דדשין בה שית
מאה אלפי גוברין בכל יומא

Therefore, it is unlikely that Rashi
or any of the other Rishonim maintain that the criterion of shishim
ribo applies to a city.

Why then does Rashi make use
of the word city in reference to the criterion of shishim ribo?

Rashi and these Rishonim are
informing us as to how cities were designed. Because most cities were walled, cities
in the past had a main road that all residents used to enter and exit the city;
therefore, this thoroughfare was the reshus harabbim of the city. Consequently,
when Rashi and the Rishonim who follow him use the word city in
reference to shishim ribo, they are not signifying that the criterion is
conditional on a city but only that the main thoroughfare [a derech hamelech]
in a city containing shishim ribo would be classified as a reshus
harabbim since it is traversed by its entire population.

This follows why Tosfos (Eruvin 6a); Rosh
(ibid., siman 8); Rabeinu Peretz (ibid., 6a); Ritva (Shabbos
6a); Ran (Eruvin 6a), and Meiri
(ibid.), when citing this Rashi (Eruvin 6a) [which utilizes city
in reference to the criterion of shishim ribo], omit the word city
because, as defined by Rashi, a city that includes a population of shishim
ribo would also contain a central corridor traversed by the entire
population of the city. A city containing shishim ribo is only an
example as to how a thoroughfare can support such a population.

Now we can understand the Rashis
in Eruvin (6b) describing the reshus harabbim of Yerushalayim and
Mechuza.

In both situations, Rashi is
referring to the main street of the city as being the reshus harabbim
and not the entire city. Yerushalayim and Mechuza contained a population of shishim
ribo which was the reason that their central routes were classified as a reshus
harabbim. Clearly the Rishonim maintain that shishim ribo is
conditional of the street and not of the city.

On the other hand, since today’s
cities are not walled, it is not necessary for the entire populace to utilize a
central thoroughfare to enter and exit the city. Hence, none of the cities’ streets
are traversed by all of its inhabitants, and therefore, even in a city with a
population greater than shishim ribo (unless 600,000 people actually
traverse the road), we would not classify any street as a reshus harabbim.

The rebuttal:First of all, there are very few later poskim who
discuss cities at all. Up until lately, it was a given that the criterion of shishim
ribo is conditional of a street (see Divrei Malkiel, 4:3). In
regards to the Admor Hazaken, how anyone can derive from this source that he maintains
that shishim ribo is conditional of a city is beyond comprehension. The
Alter Rebbe is merely indicating that (according to shitas Rashi) only a
city which has a street with shishim ribo traversing therein would
require shiyur [we exclude a section of the city from the eruv]. There
is no mention in this seif that the criterion of shishim ribo is
conditional of a city. Furthermore, it is beyond a shadow of a doubt that the
Alter Rebbe upholds shitas Rashi is conditional of a street. The Alter
Rebbe writes (363:44):

שאםהיוששיםריבואבוקעיםבמבויזההיהרשותהרביםגמורה

This
reference by the Alter Rebbe is clearly indicating that the criterion of shishim
ribo is conditional on 600,000 people traversing in the mavoi itself
and not the entire city (for additional proof see, 345:11, and 363:42).

As mentioned above, the simple understanding from the Rishonim (and Achronim)
of the criterion of shishim ribo is that it is conditional of a street. As
a matter of fact, a little further on in the same teshuvah and seif that
the Rebbe quoted, the Bais Av (2:5:3) clearly states that the criterion
of shishim ribo is conditional of a street and not of a city.